Thermoplastic vulcanizate elastomers, or TPV compositions, are conventionally produced by dynamic vulcanization. Dynamic vulcanization is a process whereby a rubber component is crosslinked, or vulcanized, under intensive shear and mixing conditions within a blend of at least one non-vulcanizing thermoplastic polymer component while at or above the melting point of that thermoplastic. See, for example U.S. patents U.S. Pat. Nos. 4,130,535, 4,594,390 and 6,147,160. Dynamically vulcanized thermoplastic elastomers (thermoplastic vulcanizates) consequently have a combination of both thermoplastic and elastic properties.
Conventional plastic processing equipment can extrude, inject, or otherwise mold, and thus press and shape TPV compositions into useful products. These thermoplastic vulcanizates can be made light in weight and attractive, with good durability, and can be reprocessed at the end of their product life to produce a new product. For these reasons, thermoplastic vulcanizates are widely used in industry, for example as auto parts, such as dashboards and bumpers, air ducts, weatherseals, fluid seals, and other under the hood applications; as gears and cogs, wheels and drive belts for machines; as cases and insulators for electronic devices; as fabric for carpets, clothes and bedding and as fillers for pillows and mattresses; and as expansion joints for construction. They are also widely used in consumer goods, being readily processed, capable of coloration as with other plastics, and providing elastic properties that can endow substrate materials, or portions thereof, for instance harder plastics or metals, in multi-component laminates, with a “soft touch” or rebound properties like rubber.
Thermoplastic vulcanizates can be prepared by dynamic vulcanization in Banbury mixers, roll mixers and other types of shearing, melt processing mixers. Because of the advantages of a continuous process, such materials can be prepared in single screw or multi-screw extruders; typically twin-screw extruders are used. Twin-screw extruders are typically available having screw diameters from 25 mm to about 380 mm. In such extruders there can be local “hot spots” created by one or both by the energy of shearing and mixing and exothermic cross-linking reactions. This can lead to some instances of polymer degradation either of the thermoplastic resin or of the rubber, unintended reactions of reactive materials, and a resulting loss of final product properties including the appearance in those products of surface spots. Also some thermoplastics can have adverse reactions, in the sense of being unwanted, with the curing agents, or cross-linking agents, for the rubber.
Surface spot, surface roughness, or “fish eyes” problems with the use of twin screw extruders have been observed when dynamic vulcanizates are proposed for sheets or surfaces. EP 1 207 034 A1 describes this problem and proposes a solution where the screw configuration is adapted for decreasing shear-derived heat generation and which is said to decrease fish eyes. Similarly EP 0 769 361 addresses the formation of small projections (fish-eyes) and bad external appearance through elevation of temperature that creates an unbalance between the dispersion and the reaction rate which is being increased by the temperature increase. Again screw configuration is proposed as a means of reducing the observed problems.
U.S. Pat. No. 6,610,786 B1 describes a thermoplastic elastomer exhibiting scarce tendency of depositing gummy crust around the extrusion die upon extrusion. The description attributes this to the process steps of reducing the amount of polypropylene (PP) present during the peroxide curing of the rubber, and subsequent introduction of additional melt-kneaded PP subsequent to the curing of the rubber.